Oxygen radicals such as O2− or O− are one type of active oxygen, and they are known to perform an important role in the oxidation process of organic substances or inorganic substances. A wide range of researches have been conducted with respect to O2− adsorbed on a solid surface of an oxide compound (J. H. Lunsford, Catal. Rev. 8, 135, 1973, M. Che and A. J. Tench, adv. Catal, 32, 1, 1983).
In such researches, O2− is prepared by irradiating the surface of an oxide compound with high energy radiation such as γ-rays.
As crystals containing O2− as constituting anions, RO2 (R=alkali metal) are known, but each of these compounds will readily undergo decomposition at a low temperature of at most 300° C. and can not be used for an application as e.g. an oxidation catalyst or an ion conductor.
In 1970, H. B. Bartl et al stated that in 12CaO·7Al2O3 (hereinafter referred to as C12A7) crystal, among 66 pieces of oxygen present in a unit cell containing two molecules, two pieces are not contained in the network and present as “free oxygen” in a space in a cage present in the crystal (H. B. Bartl and T. Scheller, Neues Jarhrb. Mineral., Monatsh. 1970, 547).
Further, Hosono et al discovered from an electron spin resonance measurement that about 1×1019/cm3 of O2− are included in C12A7 crystal synthesized by a solid phase reaction in air at a temperature of 1,200° C. using CaCO3 and Al2O3 or Al(OH)3 as starting materials, and have proposed a model such that some of free oxygen are present in a cage in the form of O2− (H. Hosono and Y. Abe, Inorg. Chem. 26, 1193, 1987).
C12A7 is a stable oxide having a melting point of 1,415° C., and it is expected to be useful as an oxidation catalyst, an ion conductor or the like, if it becomes possible to increase the amount of O2− to be included, so that reversible taking in and discharge will be possible.
Hosono et al. further conducted a study on the above C12A7 including O2−, and obtained C12A7 including O2− and O− as active oxygen species at a high concentration of at least 1020/cm3, by using CaCO3, Ca(OH)2 or CaO, and Al2O3 or Al(OH)3, as starting materials and firing them in a dry oxidizing atmosphere with an oxygen partial pressure of at least 104 Pa and a steam partial pressure of at most 102 Pa at a temperature of at least 1,20° C. and less than 1,415° C. to carry out a solid phase reaction (JP-A-2002-3218).